This invention relates generally to a biosensor for use in monitoring the concentration of target chemical analytes present in an aqueous biological system. More particularly, the invention relates to a biosensor for measuring the concentration of one or more analytes in a transdermally extracted sample. The invention also relates to an electrode system for continual transdermal extraction of one or more analytes from a biological system over an extended period of operation. One important application of the invention involves a sampling system for monitoring blood glucose using noninvasive or minimally invasive sampling techniques.
A number of diagnostic tests are routinely performed on humans to evaluate the amount or existence of substances present in blood or other body fluids. These diagnostic tests typically rely on physiological fluid samples removed from a subject, either using a syringe or by pricking the skin. One particular diagnostic test entails self-monitoring of blood glucose levels by diabetics.
Diabetes is a major health concern, and treatment of the more severe form of the condition, Type I (insulin-dependent) diabetes, requires one or more insulin injections per day. Insulin controls utilization of glucose or sugar in the blood and prevents hyperglycemia which, if left uncorrected, can lead to ketosis. On the other hand, improper administration of insulin therapy can result in hypoglycemic episodes, which can cause coma and death. Hyperglycemia in diabetics has been correlated with several long-term effects of diabetes, such as heart disease, atherosclerosis, blindness, stroke, hypertension and kidney failure.
The value of frequent monitoring of blood glucose as a means to avoid or at least minimize the complications of Type I diabetes is well established. Patients with Type II (non-insulin-dependent) diabetes can also benefit from blood glucose monitoring in the control of their condition by way of diet and exercise.
Conventional blood glucose monitoring methods generally require the drawing of a blood sample (e.g., by fingerprick) for each test, and a determination of the glucose level using an instrument that reads glucose concentrations by electrochemical or colorimetric methods. Type I diabetics must obtain several fingerprick blood glucose measurements each day in order to maintain tight glycemic control. However, the pain and inconvenience associated with this blood sampling, along with the fear of hypoglycemia, has lead to poor patient compliance, despite strong evidence that tight control dramatically reduces long-term diabetic complications. In fact, these considerations can often lead to an abatement of the monitoring process by the diabetic.
Recently, various methods for determining the concentration of blood analytes without drawing blood have been developed. For example, U.S. Pat. No. 5,267,152 to Yang et al. describes a noninvasive technique of measuring blood glucose concentration using near-IR radiation diffuse-reflection laser spectroscopy. Similar near-IR spectrometric devices are also described in U.S. Pat. No. 5,086,229 to Rosenthal et al. and U.S. Pat. No. 4,975,581 to Robinson et al.
U.S. Pat. No. 5,139,023 to Stanley describes a transdermal blood glucose monitoring apparatus that relies on a permeability enhancer (e.g., a bile salt) to facilitate transdermal movement of glucose along a concentration gradient established between interstitial fluid and a receiving medium. U.S. Pat. No. 5,036,861 to Sembrowich describes a passive glucose monitor that collects perspiration through a skin patch, where a cholinergic agent is used to stimulate perspiration secretion from the eccrine sweat gland. Similar perspiration collection devices are described in U.S. Pat. No. 5,076,273 to Schoendorfer and U.S. Pat. No. 5,140,985 to Schroeder.
In addition, U.S. Pat. No. 5,279,543 to Glikfeld describes the use of iontophoresis to noninvasively sample a substance through skin into a receptacle on the skin surface. Glikfeld suggests that this sampling procedure can be coupled with a glucose-specific biosensor or glucose-specific electrodes in order to monitor blood glucose. Finally, International Publication No. WO 96/00110 to Tamada describes an iontophoretic apparatus for transdermal monitoring of a target substance, where an iontophoretic electrode is used to move an analyte into a collection reservoir and a biosensor is used to detect the target analyte present in the reservoir.
However, there remains a need in the art for sampling devices and sampling methods which provide low cost, accurate determination of analyte concentrations in field or home-testing applications, particularly where continual and/or automatic monitoring is desired.
Accordingly, the present invention provides an efficient sampling system for detecting and/or measuring the concentration of a transdermally extracted analyte. The invention represents an improvement over prior noninvasive monitoring techniques and devices by providing an automatic sampling system coupled with a highly sensitive biosensor for determining the concentration of a target analyte present in an aqueous biological system. The sampling system extracts small amounts of a target analyte via transdermal methods, and then senses and/or quantifies the concentration of the target analyte. Sampling is carried out in a continual manner, allowing quantification to be carried out even when a target analyte, extracted from the biological system, is obtained at a sub-millimolar (sub-mM) concentration.
The advantages provided by the invention are thus several fold. For example, the noninvasive nature of the sampling system significantly increases the likelihood of patient acceptance. In the particular context of blood glucose monitoring, better glycemic control can be achieved by taking frequent blood glucose measurements on a daily basis, and using that information to determine the amount and frequency of insulin administration. Use of the noninvasive sampling system of the invention helps increase the likelihood that such frequent measurements will be taken. In addition, the automatic sampling provided by the instant sampling system, particularly when taken over an extended period of time (e.g., 24 hours or more) can be used to monitor concentration swings previously not detectable using prior devices. Again in the context of blood glucose monitoring, it is now believed that even four to seven glucose measurements per day may be insufficient to reflect the diurnal glucose level variation in many diabetics. Using the instant sampling system to automatically measure blood glucose at, for example, a frequency of once per hour, allows monitoring of previously unrecognizable glucose swings, particularly when a subject is asleep. Thus, the invention provides access to information that is of great clinical benefit in home, field and/or medical environments.
Accordingly, it is a general object of the invention to provide an automated system for continual transdermal extraction of analytes present in biological fluids. In one particular embodiment, the transdermal extraction is carried out using reverse iontophoresis or electroosmosis to extract analytes across a subject""s skin. In this embodiment, one or more collection reservoirs are contacted with a subject""s skin. The reservoirs typically contain a conductive medium and are in contact with a sampling means for providing electric potential or current between the reservoir site and another site on the subject""s skin. A biosensor is also in contact with the one or more reservoirs, and provides a means for sensing and/or quantifying the concentration of a target analyte present in the reservoirs.
In a preferred embodiment, an automated system for iontophoretic extraction of analytes is provided, wherein iontophoretic electrodes capable of continual cycling under iontophoretic conditions are used to transdermally extract analytes continually over a period of about 1-24 hours, or longer. Therefore, unlike most iontophoresis applications, the iontophoretic electrodes of the invention are capable of passing current in both directions without concomitantly participating in undesirable side reactions, particularly water hydrolysis. In addition, the electrodes must have the capacity to pass a high amount of charge, which capacity is readily reversible so that the electrodes pass current reproducibly for an extended period of operation.
In another embodiment, an automated system for continual transdermal extraction of analytes present in biological fluids is provided, wherein the transdermal extraction is carried out using sonophoresis to extract analytes across a subject""s skin. In this embodiment, a collection reservoir is contacted with a subject""s skin. The reservoir contains a conductive medium, and is in contact with a sampling means for applying ultrasound to the contacted skin surface such that noninvasive sampling of analytes beneath the skin surface can be carried out. A biosensor is also in contact with the reservoir, providing a means for sensing and/or quantifying the concentration of a target analyte extracted into the reservoir.
In each of the iontophoretic and sonophoretic sampling systems of the invention, the collection reservoirs are comprised of a liquid, or liquid-containing medium which is ionically conductive and efficiently transmits the electric potential or current, or the ultrasound, between the respective sampling means and the skin surface. In preferred embodiments, the liquid-containing medium is an ionically conductive hydrogel or wicking material soaked with an ionically conductive medium.
As will be understood by the ordinarily skilled artisan upon reading the present specification, there are a large number of analytes that can be sampled using the present automated sampling systems. In systems which rely on the reverse iontophoresis/electroosmosis techniques described herein, charged (e.g., having a negative or positive ionic charge) substances will be extracted at the highest concentrations, while uncharged substances will be extracted at lower, albeit still quantifiable, concentrations. One particular uncharged analyte of interest herein is glucose. Other analytes of interest include, but are not limited to, amino acids, enzyme substrates or products indicative of a disease state or condition, therapeutic agents, drugs of abuse, and electrolytes.
The biosensor used for sensing and/or quantitating the target analyte extracted by the present sampling system needs to perform reliably and reproducibly using extracted concentrations (e.g., sub-mM) which are well below those measured by conventional electrochemical detection (generally in the mM range). As used herein, xe2x80x9csub-mMxe2x80x9d refers to and concentration that is less than 1 mM. In one particular embodiment, the biosensor includes an electrode comprising a platinum-group metal (e.g., Pt, Pd, Ru, and Rh). The biosensor electrode is used to detect hydrogen peroxide generated by an enzyme oxidase which specifically reacts with an analyte of interest to provide hydrogen peroxide. Since the automatic sampling system is used to provide continual or periodic sampling over an extended period of operation, the biosensor electrode must have a low background current, and be stable for at least about 1-24 hours of operation. The biosensor electrode further must have high sensitivity for the hydrogen peroxide signal, where a preferred sensitivity (nA/xcexcM): background current (nA) ratio is on the order of about 3 or greater. Finally, the biosensor electrode must exhibit reduced catalytic peroxide decomposition by the platinum-group metal constituent.
Accordingly, it is a primary object of the invention to provide sampling system for monitoring the concentration of an analyte present in a biological system. The sampling system comprises:
(a) a reservoir containing an ionically conductive medium and an enzyme capable of reacting with the analyte to produce hydrogen peroxide;
(b) sampling means in operative contact with the reservoir, wherein the sampling means is used for extracting the analyte from the biological system into the reservoir to obtain a sub-millimolar (sub-mM) concentration of the analyte in the reservoir which reacts with the enzyme to produce hydrogen peroxide; and
(c) a sensor element also in operative contact with the reservoir, wherein the sensor element reacts electrochemically with the hydrogen peroxide produced in the reservoir to provide a detectable signal. The sensor element comprises an electrode having suitable geometric surface area and background noise so as to be effective in the present sampling system.
It is also a primary object of the invention to provide a sampling system for monitoring the concentration of an analyte present in a biological system, wherein the sampling system comprises:
(a) a reservoir containing an ionically conductive medium and an enzyme capable of reacting with the analyte to produce hydrogen peroxide;
(b) reverse iontophoretic sampling means in operative contact with the reservoir, wherein the reverse iontophoretic sampling means is used for extracting the analyte from the biological system into the reservoir to obtain a sub-millimolar (sub-mM) concentration of the analyte in the reservoir which reacts with the enzyme to produce hydrogen peroxide; and
(c) a sensor element also in operative contact with the reservoir, wherein the sensor element reacts electrochemically with the hydrogen peroxide produced in the reservoir to provide a detectable signal. The reverse iontophoretic sampling means comprises first and second iontophoretic electrodes having suitable geometric area and current carrying capability so as to be operative in the present sampling system.
It is a still further object of the invention to provide a method for monitoring the concentration of an analyte present in a biological system, wherein the method comprises the following steps:
(a) extracting an analyte from the biological system into a collection reservoir to provide a sub-millimolar (sub-mM) concentration of the analyte in the reservoir;
(b) contacting the analyte extracted in step (a) with an enzyme that reacts with the analyte to produce hydrogen peroxide;
(c) detecting the hydrogen peroxide produced in step (b) with a sensor element, wherein the sensor element reacts electrochemically with the hydrogen peroxide to produce a detectable signal;
(d) measuring the signal produced in step (c);
(e) correlating the measurement obtained in step (d) with the concentration of the analyte in the biological system; and
(f) performing steps (a)-(d) continually or periodically over an extended period of operation. The sensor element comprises an electrode having suitable geometric surface area and background noise so as to be operative in the present method. Optionally, the method can be carried out using a reverse iontophoretic system to transdermally extract the analyte from the biological system, wherein the iontophoretic electrodes have suitable geometric area and current carrying capability so as to be operative in the present method.
In a further aspect of the above embodiments, the sensor element can also include a reference electrode, and a counter electrode. Further, a counter electrode of the sensor element and an iontophoretic electrode of the sampling system can be combined as a single bimodal electrode where the electrode is not used simultaneously for both functions, i.e., where the counter and iontophoretic functions are separately carried out at different times.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.